Ownership and moves page lacks explanation

[BAEGregS]

I would like to open a discussion about this page: Ownership and moves

The (paraphrased) code on the page says:

// _Stack_ allocated integer
let x = 5u32;
// *Copy* `x` into `y` - no resources are moved
let y = x;

// `a` is a pointer to a _heap_ allocated integer
let a = Box::new(5i32);
// *Move* `a` into `b`
let b = a;

The comment says // _Stack_ allocated integer. I thought that u32 is copied because it implements Trait std::marker::Copy, not because it happens to be on the stack. Am I unclear on the concept?

Copy vs. move is a super-important concept, and I think that there should be an explicit explanation here of WHY let y = x; is a copy and let b = a; is a move. That might mean one short paragraph mentioning Copy and why u32 implements by default it would be good. Also, a link to the trait Copy page. Even if this information is elsewhere in the book, I still think it still should be made explicitly clear here.